The milking operation is of variable duration not only for individual cows, but also for any single cow. The milking time may thus typically vary between 3 and 10 minutes.
A dry milking process, i.e. the continued operation of a milking machine on an udder which does practically no longer yield any milk, is damaging to the teat tissue and highly dangerous with regard to the health of the udder. The milking appliance should therefore be taken off or deactivated immediately after cessation of the milk flow.
In accordance with advancing rationalization, it is nowadays customary to employ so-called milk flow indicators acting to automatically detect the end of the milking operation and to generate a corresponding signal, usually an electric or pneumatic signal. Depending on the degree of technical sophistication, the signal of the milk flow indicator results in the occurrence of one or more of the following responses:
1. An optical or acoustic signal to the operator, PA0 2. the automatic interruption of the pulsation or reduction of the milking intensity, PA0 3. the fully automatic removal of the milking appliance, PA0 4. automatic initiation of a residual milking process, or PA0 5. the control of the functional parameters of the milking machine in accordance with the actual milk flow.
The essential problems of a so-called milk flow indicator reside on the one hand in obtaining satisfactory accuracy of response, and on the other hand in the usually occurring vacuum losses. In the majority of cows, the so-called milk flow curve (milk volume per time unit) asymptotically approaches the zero flow line, although the natural configuration of the milk flow curve may be distorted by considerable variations due to the pulsation and the asynchronous intermittent milk transport.
By international agreement, the end of the milking operation is defined as the time at which the (natural) milk flow curve drops below the threshold value of 200 g/min. In the majority of cows, the intersection of the milk flow curve with this threshold value forms an extremely acute angle, so that even small measuring inaccuracies in this portion of the milk flow curve may result in considerable errors in determining the timing for the end of the milking operation.
These circumstances render the reliable determination of the actual end of the milking operation extremely difficult. To compensate for this unreliability factor, it is customary to impose a certain delay (typically about 30 sec.) between the appearance of the indicator signal and the actual initiation of the corresponding function, for instance the automatic removal of the milking appliance. In this manner it shall be ensured that the udder is in actual fact empty at the end of the milking operation, which is of considerable importance not only for economical reasons in terms of volume and fat content of the milk, but also in view of udder health. On the other hand, this delay may result in a prolongation of the prejudicial dry milking process, and in addition to a prolongation of the milking time, which leads to a lowering of operational productivity.
Milk flow indicators of the conventional type are moreover unsuitable in any case for obtaining any useful indication with regard to the magnitude of greater milk flow volumes (about 500 g/min and above), so that an efficient control of the functional parameters of the milking machine hither-to requires the employ of complicated milk volume measuring systems.
With regard to the problem of vacuum losses, it is to be kept in mind that the modern standard milking machine has to perform a twofold function: On the one hand, the vacuum is used to extract the milk from the udder by overcoming the resistance of the teat sphincter, and on the other hand the vacuum must be able to transport the thus extracted milk from the udder through the so-called long milk hose to the milk header or the measuring cup, whence the milk flows off by gravity. This transport of the milk to the milk header with the aid of the vacuum results in considerable hydrodynamic losses (flow losses), which increase in proportion to an increase of the milk flow. Inasmuch as many milking installations are designed as so-called high line installations, with the milk header installed overhead for technical and functional reasons, the extracted milk has to be conveyed upwards to a considerable height, i.e. to about 1.2 meters in the case of a milking box, and up to 2 meters in the case of a stable milking installation. The resulting additional hydrostatic losses (pressure losses) are likewise increasing in proportion to the milk flow.
Worldwide scientific endeavours directed over many years towards optimization of the functional parameters of milking machines notwithstanding, the cumulative flow and pressure losses continue to present a serious problem. This is because from the viewpoint of milking technology, the strength of the milking vacuum actually applied at the location of the udder is progressively reduced as the milk flow increases, i.e. at the very time when it would be most urgently required for efficient removal of the extracted milk, and that even in the case of a perfectly stabilized operating vacuum in the milk header (large cross-sectional passage areas, high-performance vacuum pump, accurately functioning vacuum control valve etc.). In order to be at all able to cope with the higher milk flow yields of modern cows, it is therefore indispensable to adjust the operating vacuum in conventional milking installations to a value considerably above that biologically required for the milk extraction. As a result thereof, however, the teat tissue is subjected to the unresticted prejudicially high operating vacuum as soon as the milk flow decreases during the milking operation, which results in a decrease of the vacuum losses, and particularly during the dry-milking phase, resulting in corresponding tissue damage and in the long run, to an increasingly slower milk extraction due to hardening of the teats as a biological counter-reaction of the organism.
Since a milk flow indicator can perform its function in the proper manner only when installed at a location downstream of the udder and upstream of the milk header--generally at the end of the long milk hose--it is evident from the above that any vacuum loss additionally caused by an indicator necessarily exerts a negative influence, which can moreover not be compensated, on the quality of the milking operation, with the corresponding consequences with regard to udder health and operational economy.
There has already become known a great variety of milk flow indicators.
The so-called chamber indicators comprise a closed accumulator vessel to be filled from above. Mounted within the vessel is an open-top standpipe communicating with a drain conduit therebelow and formed at the bottom level with a small calibrated drain opening permitting a continuous flow of 200 g/min into the drain conduit during the milking operation. The accumulator vessel houses a float, or optionally a pair of electrodes, for a conductive or capacitive measuring operation. It is also possible to povide a light barrier operable to supply a signal as soon as the liquid within the accumulator vessel has dropped to below a certain level, cf. for instance DE-OS 21 34 976 or U.S. Pat. No. 4,714,048.
Accumulator chambers basically permit an advantageous stabilization of the usually unstable milk flow signal to be achieved due to the integrating effect of the accumulator chamber volume. At the same time, however, the chamber volume results in the automatic occurrence of a delay, which may prove to lead to considerable disadvantages, inasmuch as the time of the delay is determined not only by the accumulator chamber volume, but also by the difference between the inflow into and outflow from the accumulator vessel, which in the case of a slowly decreasing milk flow involves the danger of extremely long delays which could in practice amount to a multiple of the actually intended value.
In indicators of the deviation chamber type, as disclosed for instance in DE-OS 22 00 141, the milk flows into the chamber from below, usually through an inflow pipe partially projecting into the chamber, internally of which the milk flow is divided. The milk flows off through an opening in the bottom or a lateral opening at the bottom level. A pair of electrodes is usually provided adjacent the bottom. In the course of the normal milking operation, the electrodes are intensively sweeped by the milk flow, so that the electric contact between the electrodes is closed. As the milk flow gradually decreases, the electrodes are swept thereby at increasing intervals, resulting in an increase of the electric resistance, until the electrodes fall completely dry and the contact is effectively interrupted.
An indicator of this type supplies a rather unreliable base signal of considerably variable strength. In addition, indicators of this type are highly sensitive to positioning irregularities, and suffer from the serious advantage of considerable vacuum losses in common with all chamber systems.
The so-called pipe indicators usually comprise a short pipe section with the milk flow usually passing therethrough from top to bottom. Since indicators of this type are usually also devoid of any flow-hampering internally mounted components, the resultant flow losses, and in particular vacuum losses, are relatively small. Known from GB-65 01 199 and U.S. Pat. No. 3,115,116 are ring electrode indicators comprising two electrically conductive pipe sections separated from one another by an insulating section in the milk flow direction. The electric resistance between the electrodes varies in proportion to the volume of the milk flow, these variations being measured. The accuracy of this measuring system is strongly affected by the varying electric conductivity of the milk of different cows, by the wide variations of the resistance values caused by the decreasing milk flow towards the end of the milking operation, and by the effect of periodical cleaning operations resulting in variations of the transfer resistance and the wettability characteristics of the pipe walls.
Known from U.S. Pat. No. 4,010,715 is a ring electrode indicator provided with electrodes on opposite sides of a conduit through which the liquid flow to be measured is directed. The application of a high-frequency AC voltage to these electrodes results in the generation of an electric current passing through the liquid and varying in accordance with the ionic conductivity of the liquid to be measured. A measuring apparatus of this type is unsuitable for measuring milk flows, since the conductivity of the milk varies from one cow to another, and is also affected by the fodder ingested by the cows.
In the case of a milk flow indicator known from U.S. Pat. No. 4,348,984, the measuring output signal is also affected by the electric conductivity of the milk flow to be measured. The milk flow measuring operation is carried out by causing the milk to flow through a coil energized by a high-frequency oscillator, resulting in the generation of an induction signal in the coil.
In photoelectric pipe indicators of the type as known for instance from EP 0 221 733, the measuring system usually comprises a short length of a smooth transparent pipe interposed in the long milk hose, usually in the vertical position, so that the milk flow passes therethrough from to bottom. A light source of a constant output intensity and a photosensor are disposed on opposite sides of the pipe. The milk flow is measured by sensing the decrease of the light refraction resulting from the decrease of the thickness of the milk film in the pipe as the milk flows therethrough. The obvious advantages of this system (no vacuum losses, insensitiveness to conductivity variations of the milk)are put into question by serious disadvantages. The accuracy of response is insatisfactory, because the light refraction is insufficient as a quantitative characteristic for accurately recognizing the thickness of the milk film. Even an extremely thin and no longer flowing milk film does still cause a considerable light absorption, with the resultant detection signal being scarcely different from that obtained from a wall flow of substantially greater thickness. This essential disadvantage is still aggravated by the fact that the measured value has to be derived from a discontinuously flowing milk and air mixture.
In "Agrartechnik" 20/2, February 1980, Trebus, Wehowsky and SchuIze proposed to eliminate the difficulties arising from the presence of milk foam and variations of the milk fat content by preventing the flow of milk foam or the formation of a milk film of a high fat content on the pipe wall of the milk flow indicator towards the end of the milking operation by the provision of beads or ribs projecting inwards from the wall of the pipe upstream of the milk flow measuring location, to act on the milk film flowing along the pipe wall in the manner of a baffle so as to keep the part of the pipe wall adjacent the location of the measuring light beam essentially free of this film. The rather substantial inaccuracy of the operation of these milk flow indicators functioning on the photoelectric principle is evidenced by an investigation published in "Tierzucht" 42, (1988, page 11). according whereto the employ of milk flow indicators of this type results in the average milking time per cow being prolonged by 2.53 min, i.e. by about 50% as compared to the case of an accurate threshold value detection with the aid of a milk volume measuring device.
It is therefor an object of the present invention to provide a method an apparatus of the type defined in the introduction, by the employ of which it is possible to obtain a more accurate measurement of the milk slug mass and on the base thereof of the milk flow and, if need be, of a milk flow threshold value even in the lower milk flow range.